Emerging epidemiological and laboratory-based research has documented that the combined consumption of alcohol with energy drinks (AEDs) leads to unique and potentially serious health risks. Specifically, adding caffeine to alcohol increases the quantity of alcohol consumed and decreases subjective feelings of intoxication, however objectively-measured blood alcohol concentration and cognitive/motor impairment remain unreduced. Consumption of energy drinks has risen markedly in the U.S. in the past decade with hundreds of energy drink brands targeting young people. Rising rates of combining energy drinks with alcohol may lead to more severe alcohol-related consequences and problems, yet little is known regarding situational covariates, potential longer-term consequences, or individual risk factors of AED use. The proposed behavioral study will document the consequences of AED consumption among a multi-ethnic sample of college students (N=746), utilizing within-person data on energy drink, alcohol, and AED use. Using a measurement burst design, with 14 consecutive days of data collected in each of 4 consecutive semesters (56 days per participant), this study will address important gaps in the extant knowledge by documenting characteristics of persons, semesters, and days associated with greater risk of AED use, as well as the extent to which AED use predicts the emergence of alcohol-related problems longitudinally. Aim 1 is to test hypotheses about daily-level covariations of AED use with behavioral risks, subjective intoxication, estimated blood alcohol content (eBAC), and health effects. Aim 2 is to document short-term longitudinal consequences of AED use, compared to alcohol use alone. Aim 3 is to identify the risk factors for individuals at greatest risk for consuming AED. High quality within-person data are necessary to identify daily effects of AED use, consequences experienced by heavier users over time, and who is most at risk for use. This proposal is intended to provide essential information to support the development of an intervention program designed to reduce AED use and consequences.